Feild’s Hydraulics Calculators for gravity flow in pipes. A four piece circular slide rule used to compute gravity flow in pipes, the original "Feild's Wheel" is based on the Manning formula and was designed in 1950 by James W. Feild. FEATURES: For pipes with diameters ranging from 4 through 72 inches, laid on slopes ranging from .04 through 10.0 feet per hundred. Determines rates of flow, velocities and ratios of liquid depths to pipe diameter when the pipe is flowing less than full. Scales for determining rates of flow in 3 units of measure: --cubic feet per second --thousand U.S. gallons per minute --million U.S. gallons per day. Michael Brogan assumed the operation of Feild's Hydraulics Calculators in 2010. He is the great-grandson of James W. Feild and the son of Ann Brogan, who sold the calculators for many years.